As the use of optical signals continues to increase and the speeds of equipment employing optical signals also increase, improved fast and accurate measurement techniques are needed. Optical signal carriers, such as fiber optic cable, and various connecting, switching, amplifying, and detecting devices require testing. As a present example of measuring speed, in some test instruments measurements are made at the rate of ten thousand or more per second.
An example of an optical test instrument, namely, an optical power meter for detecting loss factors in fiber optic communications, is described in U.S. Pat. No. 5,825,516.
An example of a device to be tested is that known as the Telecom DWDM system, that relies on deep optical filter components, which often reject at 25 dB to 50 dB and more, clearly a rather wide range. In other optical instruments signal strengths vary over an even wider range, e.g., between approximately −7 dB and approximately −45 dB. Validating performance of such devices may be performed using a tunable laser source and an optical power meter. However, conventional test systems, which include an optical power meter and associated measurement instrument, usually are generally linear-ranging instruments, and the range typically changes over three (3) to five (5) ranges, e.g., decades, as signals between 25 dB and 40 dB, for example, are measured. Usually it is desirable for a signal to be measured in the proper range setting of a measurement instrument to obtain the most accurate measurement. For example, usually it is undesirable to measure a small magnitude signal at an upper range of a multiple range measurement instrument because accuracy and resolution would be rather poor; similarly a large magnitude signal would not be measured at a low rage because the likely would be off-scale, as is well known. In conventional linear-ranging test systems although accurate measurements can be obtained, the time it takes to determine the correct range and to switch to that range can be a major component of the time required to perform the test and, thus, slows down the overall measurement procedure.
In conventional linear ranging measurement circuitry and in software ranging measurement circuitry, usually time is wasted determining the range in which the measurement circuitry should operate to measure signal strength of a given input signal. In a typical case the measurement circuitry measures the input signal using one range, usually that one range is whatever range was used for the last measurement made. The circuitry determines whether the signal is over range, i.e., larger than the measurement circuitry is able to measure while in the present range, and in such case, the ranging mechanism must select a higher range. Similarly, if the measurement demonstrates that the measured signal is below a prescribed level or percent of the signals typically measured in the present range of the measurement circuitry, e.g., below 10% of the largest signals measured at the present range, then a lower range must be selected. After selecting the new range, another measurement is made. If that measurement is in range, the measurement result is acceptable; but if that measurement shows the measured signal is above or below range, as was mentioned before, the range selection step must be repeated until an acceptable range is identified. To make a measurement in a new range, the measurement circuitry must “settle” so that the measurement circuitry is set with the newly selected range of measurement capability. Thus, total measurement time is the settling time times the number of range changes required to reach the proper range, plus the measurement time times the number of range changes. Accordingly, it is desirable to expedite the adjustments in measurement circuitry to reduce the time required for selecting range.
Measurements made using linear range measurement instruments are relatively accurate; but, as ranges must be changed, depending on the magnitude (or signal strength) of the measured signal, operation of such instruments may be relatively slow. Accordingly, there is a need to increase the speed of making such measurements.
In making measurements of signals that vary in magnitude over a relatively wide range, non-linear measurement systems, which include nonlinear amplifiers, sometimes have been used. An example of a non-linear amplifier is a logarithmic amplifier. A logarithmic amplifier based measurement instrument may be relatively faster than the linear ranging instruments mentioned above, because ranges do not have to be switched, or at least the number of switched ranges is fewer than for a linear ranging measurement instrument. However, logarithmic amplifiers compress the results of the measurement, which leads to a reduction in accuracy of the measurement, especially when measuring relatively smaller signals in the large range over which a signal magnitude or signal strength may vary. Accordingly, there is a need to improve the accuracy of such measurements.
Linear ranging measurement devices tend to be more sensitive and/or accurate than non-linear measurement devices; and non-linear measurement devices tend to be faster than linear ranging measurement devices. There is a need for increasing the speed of making measurement while maintaining a high level of sensitivity and accuracy.
The dynamic range of a measuring instrument having multiple ranges depends on the ranges. As the number of ranges increase and or the span of respective ranges increases, a reduction in speed and/or accuracy may occur due to the time constants in the ranging and/or measurement circuits. For example, the signal input or source has characteristic resistance and capacitance, which provides an “input time constant;” and the respective parts, e.g., amplifiers, of the ranging circuitry have respective characteristic resistance and capacitance, which also provides a time constant. The different ranging subcircuits in the ranging circuitry may have different respective time constants, which makes compensation for those differences as ranges change difficult, if not impossible. Absent compensation for the time constants, measurements may be taken of signals before a signal has stabilized, for example. Thus, there is a need to increase the speed of measurements made over relatively wide ranges. Likewise, there is a need to increase the dynamic range over which measurements can be made relatively fast.
Stated another way, a speed and accuracy limiting factor on high speed measurements is the time constants due to the source or input resistance and capacitance and the resistance and capacitance of respective amplifiers in the measuring and/or ranging circuitry. Due to such time constants, signals may not be adequately stable or formed for accurate high speed measurement thereof as the signals vary and/or as ranges are changed. Thus, there is a need to maintain accuracy and speed of such measurements.